Abstract: B3.00001 : Evidence for a Super-massive Black Hole at the Center of the Milky Way

Author:

Mark Reid
(Harvard-Smithsonian Center for Astrophysics)

While the concept of a black hole formed by the explosive
collapse of a dying star is astounding, the possibility that matter
from billions of stars can condense into a single
super-massive black hole (SMBH) is even more fantastic.
Yet astronomers are now confident that they exist at the centers of
most galaxies and hold more than 0.01\% of the baryonic mass of the Universe.
Early evidence for SMBHs came from ``radio galaxies'' with
two lobes symmetrically placed about the parent galaxy.
These lobes are immense and {\it minimum} energy estimates require
the total conversion of $10^7$ stars to energy! The source of energy
was traced to galaxy's center and observed to vary on time scales
$<1$ year. Since nuclear reactions convert less than 1\% of mass
to energy, this would require channeling $>10^9$ stars through a region
smaller than that between the Sun and the nearest star.
A very compact radio source was discovered toward the center
of the Milky Way and named Sgr A*, leading to speculation that it might be
a SMBH. Infrared observations of stars on elliptical orbits give clear
evidence of an unseen gravitational source of $4\times10^6$ solar masses.
One star has been seen moving at 5000 km/s in its 16 year eccentric orbit.
Sgr A* has been located at the position of the gravitational focus
of the stellar orbits. However, in contrast to the rapidly moving stars,
Sgr A* is motionless ($<1$ km/s), requiring the source to be extremely
massive. For comparison, gravitational ``Brownian motion'' of a SMBH
at the center of a dense stellar cluster would be comparable to the
measured limits.
Recent radio interferometric observations show that the radio emission
from Sgr A* comes from a region comparable in size to the Schwarzschild
radius ($2GM/c^2$) of 0.1 AU ($1.5\times10^7$ km)!
Placing any known concentration of $4\times10^6$ solar masses
within this tiny volume would rapidly condense to a
black hole.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2012.APR.B3.1